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MSFS20 Beginner needs help with data type

P

Phil75

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3
Country
france
Hello,

I'm starting out with programming using Simconnect and VB.net.
What I started doing is working.
But when I write this line of code:

Code: 
Structure Struct1
    Public Speed As Double
End Structure

MSFSobj.SimObjet.AddToDataDefinition(StructDefinitions.Struct1, "Ground Velocity", "knots", Microsoft.FlightSimulator.SimConnect.SIMCONNECT_DATATYPE.FLOAT64, 0, 0)

I don't know why "knots" is of type "Float64".

Regarding the Simvar "GROUND VELOCITY", the SDK documentation only indicates the description and the unit.

How do you know which VB data type corresponds to which Simconnect Unit?
 
Hi Phil,

Phil75 said:
I don't know why "knots" is of type "Float64".
Click to expand...

The data type you specify needs to be able to contain the type of value you expect or want. For numeric data types (basically everything except strings), there's no hard rule as to which SimConnect type to use, but it must match the bit size you specify as your container variable type. So in your example you use a Double size variable for Speed, which is 64 bits and therefore the SimConnect FLOAT64 data type is the correct match (both are same size and represented the same way in the bit layout).

However if you don't need 64-bit precision for that value (which you probably don't), then it would be just as valid to make Speed a Single type (32-bit floating point) and use FLOAT32 SimConnect data type.

Or if you don't care about fractions at all, you could make Speed be an Int32 type and use SIMCONNECT_DATATYPE_INT32 type.

Similar for integer type sim values or enums or booleans. You could just specify them all as Doubles (both in your storage and in the AddToDataDefinition() call), but that could waste storage space and network traffic (data sizes) and possibly slow down math or comparison operations. Instead you could use the smaller INT32 (integer) type.

Arguably, the only values from the sim that really need double precision are Lat/Lon.

Unrelated, but keep in mind you don't need a structure if you just want a single variable -- you can use/register a single variable directly w/out needing a struct wrapper. Unfortunately the official documentation doesn't show this.

Hope that helps,
-Max
 
Last edited:
Two points:

1 - Not all data types work correctly with all variables, dunno why. FLOAT64 is the most common and typically works for all sim variables.
2 - FLOAT64 is a Double, which is a floating-point value (fractional parts are supported).
 
And if FLOAT64 doesn't work (e.g. ON GROUND) then FLOAT32 does work. A loooong time ago I picked up the information that all FS internal variables are doubles (most likely from one of the ACES team but I'm no longer able to swear to that). A lot of the FS stuff has carried forward into MSFS202x as what they call 'legacy' and SimConnect is pretty much all 'legacy'.
 
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